Small magnet and RF coil for magnetic resonance relaxometry

ABSTRACT

Small and inexpensive probeheads for use in nuclear magnetic resonance systems, in particular, magnetic resonance relaxometry systems are provided. The design of the magnet-radiofrequency coil configurations within the probeheads is guided by an excitation bandwidth associated with radiofrequency pulses to be applied to a sample.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/741,789, filed Aug. 20, 2010, which is the U.S. National Stage ofInternational Application No. PCT/US2008/012592, filed Nov. 6, 2008,which claims the benefit of U.S. Provisional Application Nos.61/002,022, filed Nov. 6, 2007, and 61/008,991, filed Dec. 21, 2007, allhereby incorporated by reference.

BACKGROUND OF THE INVENTION

Nuclear magnetic resonance (NMR) systems make use of nuclear magneticresonance of atomic nuclei contained in a sample and are known to beable to provide a large variety of information characterizing the sampleand corresponding sample components. Systems include, for example,magnetic resonance imaging (MRI) devices, magnet resonance spectrometersand magnetic resonance relaxometers. The nature of the nuclear magneticresonance phenomenon requires the presence of a magnetic field uponexcitation with a radiofrequency electromagnetic wave. Thus, generally,NMR systems include a magnet and a radiofrequency coil, either asseparate system components or combined in a probehead.

Magnets that are preferred in magnetic resonance systems providemagnetic fields with high magnetic field strength and high homogeneity.Magnets known to satisfy these requirements are typically large and/orexpensive. They are therefore not suitable for portable devices and/orimplantation devices, and/or not suitable as part of disposableprobeheads. Thus, a need exists for small, inexpensive probeheads foruse in magnetic resonance systems, allowing portability, implantationand/or one-time use applications.

SUMMARY OF THE INVENTION

Provided probeheads and methods of preparing the same solve the problemsof the current MR systems relating to portability, potentialimplantation and/or disposability of probeheads for use in MR systems.Probeheads provided in the present invention are particularly suitable,though not limited to, magnetic resonance relaxation measurements.

One embodiment is a small probehead for use in a magnetic resonancerelaxometer. The probehead comprises (a) at least one magnet or magneticfield generator providing a magnetic field, (b) a space capable ofaccommodating a sample volume having an associated excitable volume, and(c) a radiofrequency coil having an associated detection volume, theradiofrequency coil being adapted and positioned such that its detectionvolume overlaps at least partly with an excitable volume. The providedmagnetic field is inhomogeneous, and the space accommodating the samplevolume and the radiofrequency coil are adapted and positioned accordingto a radiofrequency pulse optimized for the magnetic field distributioncorresponding to the position of the sample volume. A provided probeheadis optimized to obtain relaxometry parameters from a sample contained inthe detection volume.

Another embodiment of is a probehead for magnetic resonance relaxometry.A small probehead comprises (a) two magnets or two magnetic fieldgenerators attached to a yoke, the south pole surface of one of themagnets or magnetic field generators opposing the north pole surface ofthe other magnet or magnetic field generator to form a gap between themagnets or magnetic field generators and to provide a magnetic field inthe gap, (b) a space capable of accommodating a sample volume having anassociated excitable volume, and (c) a radiofrequency coil within thegap, the radiofrequency coil having an associated detection volume andbeing adapted to emit a radiofrequency pulse with a pulse length, theradiofrequency coil being positioned and designed to have the detectionvolume partly overlap with an excitable volume within the gap. Theprovided magnetic field is inhomogeneous. Additionally, the spaceaccommodating the sample volume and the radiofrequency coil are adaptedand positioned according to a radiofrequency pulse bandwidth optimizedfor the magnetic field distribution corresponding to the position of thesample volume. The probehead is thus optimized to obtain relaxometryparameters from a sample contained in the sample volume.

Additionally provided are methods for preparing probeheads for use in amagnetic resonance relaxometry. In one embodiment is a method forpreparing a probehead for use in a magnetic resonance relaxometer. Themethod comprises the steps of (a) providing at least one magnet ormagnetic field generator providing a magnetic field, (b) providing aradiofrequency coil, (c) positioning the radiofrequency coil to have itsassociated detection volume overlap at least partly with an excitablevolume, (d) positioning a space capable of accommodating a sample volumehaving an associated excitable volume; and (e) adapting the space forthe sample volume and the radiofrequency coil according to aradiofrequency pulse optimized for the magnetic field distributioncorresponding to the position of the detection volume. The probehead isthus optimized to obtain relaxometry parameters from a sample containedin the sample volume.

A further embodiment is a method of preparing a small probehead for usein portable magnetic resonance relaxometry. The method comprises thesteps of (a) attaching two magnets or two magnetic field generators to ayoke such that the south pole surface of one of the magnets or magneticfield generators opposes the north pole surface of the other magnet ormagnetic field generator to form a gap between the magnets or magneticfield generators and to provide a magnetic field in the gap, (b)positioning a space capable of accommodating a sample volume having anassociated excitable volume, and (c) positioning a radiofrequency coilwithin the gap, the radiofrequency coil having an associated detectionvolume and being adapted to emit a radiofrequency pulse with a pulselength, the radiofrequency coil being positioned and designed to havethe detection volume at least partly overlap with an excitable volumewithin the gap. The probehead is thus optimized to obtain relaxometryparameters from a sample contained in the sample volume.

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic representation of a probehead including ac-shaped yoke with magnets attached thereto and a radiofrequency coilplaced between the magnets.

FIG. 2 shows a probehead employing two NdFeB permanent magnets and aten-turn radiofrequency coil from two sides.

FIG. 3 shows the “T2-yoke” made from a steel yoke and 1″×1″×0.5″ NdFeBmagnets.

FIG. 4 shows a Halbach magnet positioned on a field mapping apparatuswith a gaussmeter probe positioned within the center gap.

FIG. 5 provides a schematic representation (side view and front view) ofa probehead including a c-shaped yoke with magnets attached thereto anda radiofrequency coil placed between the magnets.

FIG. 6 shows the measured dependency of the magnetic field strengthalong three directions (along the gap, across the gap, and from bottomto top) within the gap between the magnets of the probehead shown inFIG. 2.

FIG. 7 provides a shaded field map within the center gap of atheoretical model magnet.

FIG. 8 provides a relaxation decay curve of a nanoparticle assaymeasured using the magnet and probehead in FIG. 2. A solution ofmagnetic relaxation switch nanoparticles sensitized to the protein β-hCGwas used to detect a concentration of 65 nM (or 1 microgram/mL) hCG in0.4 microliters of pH 7.4 PBS buffer.

FIG. 9 shows the measured dependency of the magnetic field strengthalong three directions (along the gap, across the gap, and from bottomto top) within the gap between the magnets or within the center gap ofthe T2-yoke magnet (shown in FIG. 3) and the Halbach magnet (shown inFIG. 4) respectively. Examples of determining the region that can beexcited for each dimension are shown with the dashed-line boxes.

FIG. 10 shows the radiofrequency coil resonant circuit for the Halbachmagnet.

FIG. 11 shows the radiofrequency coil resonant circuit for the T2-yoke.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THE INVENTION

Probeheads of the present invention include a magnet and/or magneticfield generator and at least one radiofrequency coil, and are muchsmaller and much less expensive than conventional combinations ofmagnet(s) and radiofrequency coil(s).

Weight and size of a probehead are critical factors for portable MRinstruments. For example, weight and size reduction has implications inregards to system development and manufacturing, cost, and placement.Small probes may be, e.g., implantable in-vivo devices, embedded sensorsfor material testing, and sensors for on-line process monitoring.Additionally, because they are inexpensive provided small probeheads maybe used in applications that benefit from disposable probeheads.

One aspect of the present invention is the scalability of a MagneticResonance (MR) probehead comprising a magnet and a radio-frequency (RF)coil. In particular, the present invention addresses the issue ofsignificantly reducing size of probehead components while allowingmeasurement of magnetic resonance signal level(s), and, in particular,magnetic resonance relaxation parameter(s) and time(s). Designing aprobehead specifically for relaxometry instead of conventional MRspectroscopy, allows for a dramatic reduction in its size and cost.

Magnet configuration and yoke design, if desired, can be accomplishedinitially by a theoretical prediction of what magnet and yokeconfiguration will lead to in terms of magnetic field strength. Suitablemagnetic field strength will be discussed below. This can be done usingstandard analytical methods known in the art.

In one embodiment at least one magnet or magnetic field generator isshaped and/or configured to provide the magnetic field in a gap. Incertain embodiments, a radiofrequency coil is positioned, either partlyor completely within the gap of such a configuration.

For a given magnet configuration, for example, two opposing permanentmagnets as presented schematically in FIG. 1 and shown in FIGS. 2 and 3,and a Halbach magnet as shown in FIG. 4, the magnetic field in the x, y,and z directions can be determined using standard methods known in theart, for example, by fixing a gaussmeter probe relative to the magnetand moving the magnet in incremental steps with a three axis stage whilerecording the field strength as a function of position to obtain a fieldmap.

Given knowledge of the magnetic field, for example, in terms of acalculated or measured magnetic field map and a pulse length of aradiofrequency pulse to be used in relaxation measurements, aradiofrequency coil or radiofrequency coil array can be designed andconcurrently a proper position for the same be determined. Pulse lengthand excitation bandwidth are inversely related. For example, a 2 μspulse corresponds to a 500 kHz excitation bandwidth (see below for moredetails). The excitation bandwidth can be used to calculate: 1) for agiven sample volume, the necessary magnetic field homogeneity to be ableto excite part of or an entire sample volume, and/or 2) for a givenmagnet or magnet array, the volume that is excitable with aradiofrequency pulse of a given pulse length in the presence of themagnetic field of the given magnet or magnet array.

Typically, a given excitation bandwidth dictates a requisite magneticfield homogeneity. Once a magnet is designed to create limitedhomogeneity of a volume that is suitable or desirable for a sample(e.g., which may be dictated by fluidics or specimen size of a sample),a coil is designed to excite a complete volume of excitable spins of asample volume. Thus, according to the present invention, an excitationbandwidth appropriate for a magnet configuration guides the magnet andcoil design as well as the probehead configuration design.

A probehead of the present invention includes (a) at least one magnet ormagnetic field generator providing a magnetic field; (b) a space capableof accommodating a sample volume having an associated excitable volume;and (c) a radiofrequency coil having an associated detection volume, theradiofrequency coil being adapted and positioned such that its detectionvolume overlaps at least partly with the excitable volume. The magneticfield provided by the magnet or magnetic field generator isinhomogenous. The space accommodating the sample volume and theradiofrequency coil are adapted and positioned according to aradiofrequency pulse bandwidth optimized for a magnetic fielddistribution corresponding to the position of the sample volume. Theprobehead is thus optimized to obtain relaxometry parameters from asample contained in the detection volume.

An “excitable volume” as used herein is a volume of hydrogen nuclei ofwater within a sample volume which are transitioned to a higher energystate by a radiofrequency pulse of a given pulse length in the presenceof a magnetic field provided by a magnet and/or magnet field generator.

All atomic nuclei with an odd atomic mass or an odd atomic number (likehydrogen nuclei of water for example) possess an intrinsic nuclearmagnetic momentum. When such atomic nuclei are placed in a staticmagnetic field, this momentum can take at least two differentorientations. For spin ½ nuclei, such as ¹H the momentum may take eithera parallel or anti-parallel orientation relative to the magnetic field.Considering a population of hydrogen nuclei immersed in the same staticmagnetic field, the number of nuclei having a parallel orientation isslightly greater than the number of nuclei having an anti parallelorientation (a ratio of 1,000,003:1,000,000 at fields of 0.5 T and roomtemperature). This is due the fact that the parallel orientation is onlyslightly more energetically favorable. Transitions from a parallel stateto an anti-parallel state occur when nuclei absorb electromagneticenergy at a given frequency called a resonance frequency, which isdictated by the strength of the magnetic field. Typically, hydrogennuclei in different locations in a magnetic field experience differentmagnetic field strengths and therefore have different resonancefrequencies required for excitement. Therefore, in prior systems, arange of frequencies were necessary to sufficiently excite a significantportion of hydrogen nuclei in a sample and generate effective relaxationreadings. A given pulse length produces a corresponding excitationbandwidth that, at a given magnetic field, excites a volume of hydrogennuclei with a radiofrequency pulse. The resulting signal afterexcitation can be detected via typical methods known in the art.

In one embodiment, a RF coil included in a probehead of the presentinvention is adapted to provide pulse lengths between about 0.4 μs andabout 10 μs. Typically, a pulse length of between about 0.5 μs and about4 μs is used. More typically, a pulse length of between about 1 μs andabout 4 μs is used. Even more typically, a pulse length of between about1 μs and about 3 μs is used.

A “probehead” as used herein is a sensing or probing device of a nuclearmagnetic resonance system. A probehead may be implanted, partially orcompletely, in a mammal's body. Typically, a probehead of the presentinvention includes (a) at least one magnet and/or magnetic fieldgenerator providing a magnetic field, and (b) a radiofrequency coilhaving an associated detection volume, and the radiofrequency coil beingpositioned such that its detection volume overlaps at least partly withan excitable volume.

In one embodiment, a probehead comprises a space capable ofaccommodating a sample volume and/or a port. In certain embodiments, aspace capable of accommodating a sample volume and port can be, forexample, a radiofrequency coil (as part of a radiofrequency circuit)wound to enclose a sample volume while providing an opening (i.e., spacecapable of accommodating a sample volume) to allow a sample volume to beplaced within the opening. In other embodiments, a space capable ofaccommodating a sample volume and/or port is distinct from the openingof a radiofrequency coil but adapted to a given radiofrequency coil, forexample, formed to enclose part or all of a detection volume of theradiofrequency coil. For example, a glass capillary within aradiofrequency coil.

In some embodiments a radiofrequency coil is wound to enclose a volumeof less than about 500 μl. In certain embodiment a radiofrequency coilis wound to enclose a volumes of less than about 100 μl. In still otherembodiments a radiofrequency coil is wound to enclose a volume of lessthan about 10 μl are used. In still further embodiment a radiofrequencycoil is wound to enclose a volume of less than about 5 μl. In particularembodiments a radiofrequency coil is wound to enclose a volume of lessthan about 1.6 μl. In still further particular embodiments aradiofrequency coil is wound to enclose a volume of less than about 0.4μl.

Also, for implantable probeheads, typically, material used to form asample volume, and, in particular, any material that may be in contactwith a biological sample or tissue is typically biocompatible, that isconstructed of materials that allow for proper function of both thedevice and a host animal's biological functions and/or coated with aphysiologically acceptable coating as known in the art to render theimplantable bioinert, biomimetic, or bioactive, as desired. Suitablematerials include titanium, inert silicone elastomers, ceramics, glass,polymeric materials, poly-β-hydroxybutyrate (PHB) and the like. One ormore sample volumes and corresponding ports can be fabricated usingmethods known in the art. Suitable methods include form or injectionmolding methods, and microfabrication methods for sample containerssmaller than a few millimeter, for example, two-photon three-dimensionallithography. A probehead may contain a “housing” that encloses thecomponents of the probehead such as, for example, a radiofrequency coiland magnet. In certain embodiments at least one component of a probehead(e.g., a magnet, a magnetic field generator, a radiofrequency coil) isattached to the housing.

A “port” as used herein, refers in the simplest case to an opening asprovided above, but can also be a structure or device that is adapted toselectively allow analytes or reagents to enter and/or exit the samplevolume.

In certain embodiments, a probehead includes one or more separate samplevolumes. In some embodiments a probehead includes between about 1 andabout 100 sample volumes. In some embodiments a probehead includesbetween about 1 and about 10 sample volumes. In some embodiments aprobehead includes two sample volumes. In certain embodiments aprobehead includes one sample volume.

A probehead containing more than one sample volume may comprise aradiofrequency coil with an associated detection volume encompassing atleast part of each sample volume. Alternatively, a probehead may havemore than one radiofrequency coil and/or radiofrequency circuit, one foreach sample volume or a subgroup of the sample volumes. In certainembodiments, a probehead comprises at least two radiofrequency coils.Also, a probehead of the systems of the present invention can include amagnet or magnetic, field generator as discussed above.

For probeheads that include a plurality of separate sample volumes butonly one radiofrequency coil that is employed to probe the plurality ofsample volumes simultaneously, multiplexing methods may be used todistinguish the magnetic resonance signal or information from theseparate sample chambers. For example, one multiplexing method that maybe used is based on extracting decay constant values, for example,values of spin-spin relaxation constant T₂ from multi-exponentialrelaxation curves (see T. J. Lowery et al., Anal. Chem. (2008), 80,1118-1123.). Relaxation data obtained using a probehead of the presentinvention may be fit to a decaying exponential curve defined by thefollowing equation:

${{f(t)} = {\sum\limits_{i = 1}^{n}\;{A_{i}{\exp\left( {{- t}\text{/}{T(i)}} \right)}}}}\;$where ƒ(t) is the signal intensity as a function of time, t, A_(i) isthe amplitude coefficient for the ith component, and (T)_(i) the decayconstant (such as T₂) for the ith component. For relaxation phenomenondiscussed here the detected signal is the sum of a discrete number ofcomponents (i=1, 2, 3, 4 . . . n). Such functions are called mono-, bi-,tri-, tetra- or multi-exponential, respectively. Due to the widespreadneed for analyzing multi-exponential processes in science andengineering, there are several established mathematical methods forrapidly obtaining estimates of A_(i) and (T)_(i) for each coefficient(Istratov, A. A. & Vyvenko, O. F. 1999. Exponential analysis in physicalphenomena. Rev. Sci. Inst. 70 (2): 1233-1257).

A “magnet” as used herein can be any material or combination ofmaterials that provides a magnetic field in at least some volume aroundthe material. Typically, the magnet is a permanent magnet. Suitable,materials include but are not limited to NdFeB, FeCo, and the like.Magnets can be configured to form new magnets, that is, magnet arrays,for example, a permanent magnet with a c-shaped yoke, a Halbach magnet(cylinder and other configurations), u-magnet, torroidal magnet and thelike.

The magnets, magnet configurations and magnetic field generators of thepresent systems can be weak and/or provide magnetic fields that areinhomogeneous. Typically, maximum magnetic field strength valuesprovided by the magnets and/or magnet configurations of the presentinvention are between about 0.2 Tesla and about 2 Tesla. More typically,they are between about 0.3 and about 1.5 Tesla. Even more typically,they are between about 0.4 and about 1.1 Tesla. Even more typically,they are between about 0.2 and about 1.1 Tesla. Even more typically,they are between about 0.2 and about 0.8 Tesla. Most typically, they arebetween about 0.45 and 0.85 Tesla. In some embodiments the magneticfield strength is less than about 2 Tesla. In certain embodiments themagnetic field strength is less than about 1.1 Tesla. In certainembodiments the magnetic field strength is less than about 0.8 Tesla.

The term “inhomogeneous” refers to magnetic fields that are lower inuniformity than those required for spectroscopy. Homogeneity isdependent on the space in which the measurement is defined. For theinstant applications, homogeneities of the magnetic fields can rangebetween about 10000 ppm and about 10 ppm. in some embodimentshomogeneities can range between about 50 ppm and 5000 ppm. In particularembodiments homogeneities can range between about 100 ppm and about 1000ppm.

Also, typically, magnetic fields employed in the present systems areeffectively static, that is, they do not change substantially over time.Changes in magnetic field such as due to temperature fluctuations areconsidered to be not substantial.

Small probeheads of the present invention can be used for, but are notlimited to in-vivo magnetic resonance measurements. Small probeheads forcomplete implantation within a mammal's body, preferably have smallmagnets to lessen the invasiveness of the implantation. Typically,magnets for implantation are smaller than about 2 inches in anydimension. More typically, magnets for implantation are smaller thanabout 1 inch in any dimension. Most typically, magnets for implantationare smaller than about 0.5 inches in any dimension.

The probeheads of the present invention can also be used in-vitro, forexample, as part of small and/or portable magnetic resonance systems.Typically, magnets in probeheads for these systems are smaller thanabout 2 inches in any dimension. More typically, they are smaller thanabout 1 inch in any dimension. Most typically, they are smaller thanabout 0.5 inches in any dimension. Each dimension may be independentlydetermined.

A “magnetic field generator” as used herein, is a device that provides amagnetic field in at least some volume around the device. Typically, amagnetic field generator requires a power supply and provides thetargeted magnetic field only when powered. Examples of magnetic fieldgenerators include but are not limited electromagnets with and without ametal pole (see Cardot et al Sensors and Actuators 1994).

Probeheads using magnetic field generators can be implanted in amammal's body. However, because magnetic field generators tend to belarger than magnets, and they are more complex, for example, require apower supply, more typically, probeheads using magnetic field generatorsare used for disposition outside a mammal's body.

The magnet(s) and magnetic field generator(s) in the present systems areselected and positioned to provide a magnetic field of sufficientstrength in the sample volume to allow measuring magnetic resonancesignals. The magnetic field strength of a given magnet or magnetic fieldgenerator in a given volume, for example, a sample volume can becalculated and/or approximated using methods known in the art.Typically, the magnetic field strength depends on the nature of themagnet or magnetic field generator and the position of the magnet ormagnetic field generator relative to the sample volume. Also, magneticfield strength of a given magnet or magnetic field generator in a samplevolume can be measured using methods and devices known in the art, forexample, gaussmeters, teslameters, hall effect probes, and the like.Typically, magnetic field strengths within a sample volume of betweenabout 0.2 and about 2 Tesla are sufficient to allow measuring magneticresonance signals. More typically, magnetic field strengths within thesample volume of between about 0.2 and about 1 Tesla are sufficient toallow measuring magnetic resonance signals. Even more typically,magnetic field strengths within the sample volume of between about 0.2and about 0.8 Tesla are sufficient to allow measuring magnetic resonancesignals. Most typically, magnetic field strengths within the samplevolume of between about 0.3 and about 0.65 Tesla are sufficient to allowmeasuring magnetic resonance signals.

The magnets and magnetic field generators suitable for the probeheads ofthe present invention are not limited to any particular size. However,in particular, for implantable and handheld probeheads small magnets aredesired. Typically, each of the at least one magnet or magnetic fieldgenerator of the probeheads of the present invention is in any dimensionless than about one two inches. More typically, each of the at least onemagnet or magnetic field generator is in any dimension less than about 1inch. Most typically, each of the at least one magnet or magnetic fieldgenerator is in any dimension less than about 0.5 inch.

Probeheads of the present invention may be used to sense/measuremagnetic resonance signals as part of a magnetic resonance system withsensing reagents enclosed within the probehead, and, in particular,within one or more sample volume.

A “sensing agent” as used herein is an agent that senses, responds to oris influenced by a sample characteristic to correlate the presenceand/or extent of the sample characteristic with the presence, change ormagnitude of the magnetic resonance signals associated with the sample.The term “sample characteristic” as used herein refers to any chemicaland/or physical property of a given sample. Suitable samplecharacteristics can be, but are not limited to concentration of ananalyte (that is, a molecule, ion, or radical of interest in thesample), pH-value, ionic strength, hydration state (e.g., of tissue orbiofluids, that is, concentration of water in tissue or biofluids),temperature, and the like.

Suitable sensing agents can be, but are not limited to dry reagentcompositions, magnetic particles, responsive polymers, magneticresonance contrast agents, and the like.

Dried reagent compositions that are suitable include, for example, driedbiotinylated coated nanoparticles (see T. J. Lowery et al., Anal. Chem.(2008), 80, 1118-1123), for example, based on the following formulation(216 μL, 0.083 mM Fe, 10 mM PBS, 20 mg/ml dextran, pH 7.4). Driedreagent compositions can be prepared by placing a magnetic particlesolution, for example, biotinylated coated nanoparticle solution into acontainer, for example, a container such as a glass tube, and freezingthe container in a freeze dryer (e.g., VirTis freeze dryer (Gardiner,N.Y.)), for example, at −80° C. for 24 h. Each of the one or moreseparate volumes of the sample containers may be filled by transfer ofthe dried reagent composition from the container that was used duringfreeze drying.

“Magnetic particles” as used herein, are particles that respond to orare influenced by a sample characteristic to correlate the presenceand/or extent of the sample characteristic with the presence, change ormagnitude of the magnetic resonance signals associated with the sample.Typically, the magnetic particles respond by aggregating. Also,typically, magnetic particles have an average particle size of betweenabout 1 nm and 5 μm. Magnetic particles may be paramagnetic orsuperparamagnetic. They can have binding moieties on their surface. Thebinding moieties are preferably operative to alter the aggregation ofthe magnetic particles as a function of the presence or concentration ofthe analyte. The magnetic particles may include an oxide and/or ahydroxide of Fe, Si, Sn, An, Ti, Bi, Zr, and/or Zn. The magneticparticles are preferably superparamagnetic and have crystallite sizefrom about 1 nm to about 100 nm. The magnetic nanoparticles preferablyhave a metal oxide core of about 1 to about 25 nm, from about 3 to about10 nm, or about 5 nm in diameter. The binding moieties may include oneor more species of one or more of the following: an amino acid, anucleic acid, an oligonucleotide, a therapeutic agent, a metabolite of atherapeutic agent, a peptide, a polypeptide, a protein, a carbohydrate,a polysaccharide, a virus, and/or bacteria. For example, in oneembodiment, the binding moieties may include one, two, or more types ofoligonucleotides and/or one, two, or more types of proteins. The bindingmoieties may be a polymer, or may be part of a polymer that is linkedto, or otherwise associated with one or more of the magnetic particles.The binding moieties preferably include functional groups, for example,the binding moieties may include one or more species of one or more ofthe following: an amino group, a carboxyl group, a sulfhydryl group, anamine group, an imine group, an epoxy group, a hydroxyl group, a thiolgroup, an acrylate group, and/or an isocyano group.

The analyte may include one or more species of one or more of thefollowing: a protein, a peptide, a polypeptide, an amino acid, a nucleicacid, an oligonucleotide, a therapeutic agent, a metabolite of atherapeutic agent, RNA, DNA, an antibody, an organism, a virus,bacteria, a carbohydrate, a polysaccharide, and glucose. The analyte mayalso include, for example, a lipid, a gas (e.g., oxygen, carbondioxide), an electrolyte (e.g., sodium, potassium, chloride,bicarbonate, BUN, creatinine, glucose, magnesium, phosphate, calcium,ammonia, lactate), a lipoprotein, cholesterol, a fatty acid, aglycoprotein, a proteoglycan, and/or a lipopolysaccharide.

For example, magnetic particles can be adapted to respond to glycatedhemoglobin. For example, amino-CLIO nanoparticles, that is, iron oxidenanoparticles coated with amino-functionalized cross-linked dextran, maybe decorated with boronate compounds by standard solution-phasechemistries. The boronate compounds such as boronic acid, phenylboronic,boric acid and boronate, etc. have an affinity for HbA1c, a specifictype glycated hemoglobin designated based on its separation from otherspecies of glycated hemoglobin. Hemoglobin is composed of four subunits,two α chains and two β chains therefore HbA1c is divalent. The divalencyallows HbA1c to facilitate the boronic acid functionalizedsuperparamagnetic iron oxide particle agglomeration. Boronate reactswith HbA1c in a sample through the cis-diol moiety of glucose bound tohemoglobin, forming a five-membered ring structure. A boronate group canbe attached to a solid phase covalently or electrostactically by avariety of chemistries. Solid phases such as amino-CLIO nanoparticlescan be decorated with boronate compounds by standard solution-phasechemistries. Amino-CLIO are iron oxide nanoparticles coated withamino-functionalized cross-linked dextran. The dextran polymer coatingendows these nanoparticles with solubility and enabled solution-phasechemistries. Suitable boronate compounds include but are not limited to4-carboxyphenylboronic acid, 3-nitro-5-carboxyphenylboronic acid, andm-aminophenylboronic acid (APBA).

“Nanosensors” are paramagnetic or superparamagnetic magnetic particles,typically of nanometer scale, that comprise a polymer matrix layer abouta magnetic core and/or are derivatized/functionalized with bindingmoieties or affinity groups for a target compound or analyte. Suitablenanosensors include responsive polymer-coated magnetic nanoparticles.These nanosensors can exploit the ability of magnetic nanoparticles todephase nuclear spins detectable by nuclear magnetic resonance (NMR),hereinafter generally exemplified as the protons of water molecules, fordetection without aggregation of nanoparticles. Each nanoparticle has apolymer matrix layer which expands or contracts when exposed to ananalyte and/or condition to be detected. The resulting change innanoparticle size affects the dephasing of freely-diffusing watermolecules in the vicinity of the nanoparticles, which affects one ormore NMR-detectable properties. By calibrating the NMR-detectedproperties with known reference samples, the existence of the conditionand/or analyte of interest may be detected in test samples via NMRtechniques using the probeheads of the present invention.

In the case where the detected nuclei are water protons, the polymermatrix preferably takes the form of a stimuli or molecule sensitivehydrogel comprising a polymer “mesh” that is cross-linked by bindingmoieties that affects the volume, permeability and the proton content ofthe matrix as a function of a physical or chemical stimulus or aphysical parameter of the analyte under study. This is accomplished bydesign of the matrix as a hydrophilic polymer network comprising (aspendent groups or as part of the polymer backbone) binding moieties thatinfluence water permeability (and/or permeability of other molecules inthe environment) through formation of one or more covalent or hydrogenbonds, van der Waals interactions, or physical entanglement with acomponent of the analyte. The presence of analyte induces a change inthe crosslink density of the polymer, which leads to a change in thevolume fraction of the solution occupied by the polymer. The change incross link density also leads to a change in the diameter of thenanoparticles, which leads to a change in their diffusion time. Bothdiffusion time and specific volume are proportional to the T₂ relaxivityobserved for a solution, as shown in the proportionality:1/T ₂α(V _(p))(R ² /D)where V_(p) is the specific volume fraction of the particles insolution, R the radius of the particles, and D the diffusion constant ofwater. The term R²/D is equal to the diffusion time, τ_(d). This is thetime necessary for a water molecule to diffuse past a particle, and isproportional to the extent of T₂ relaxation that occurs.

The binding moiety may be a chemical binder, an electroactive mediator,an electron-pair donor, and/or an electron-pair acceptor. It may containan amino, carboxyl, sulfhydryl, amine, imine, epoxy, hydroxyl, thiol,acrylate, or isocyano group, or a mixture thereof. For example, thebinding moiety may be an acetic acid moiety such as in poly(acrylicacid) for sensing pH, or phenylboronic acid for sensing the presence ofdiols, such as glucose Alternatively, the binding moieties are bindingpairs, or binding pendants, such as antibodies that serve ascross-linkers in the presence of their cognate antigen, or antigens thatserve as cross-linkers in the presence of their cognate antibodies, andwhich mediate the water proton flux in and out of the matrix and changein specific volume by competitive affinity reactions. This typically isaccomplished as the extent of cross-linking of matrix polymer ismediated as a function of the physical parameter under study so as tocontrol the permeability of water, including its amount and rate oftranslational diffusion in an out of the matrix and within the matrixvolume in proximity to the magnetic particle(s). For example, thebinding pairs may be a ligand binding protein such as concanavalin Abound to a low-affinity ligand such as a carbohydrate. Addition ofglucose to this system would displace the low affinity ligand and changethe crosslinking of the matrix. Another example is a matrix-immobilizedantibody, antibody fragment, or peptide that crosslinks the matrix bybinding to its matrix-immobilized antigen or target. The presence of ahigher affinity analyte would lead to disruption of the cross-linkedmatrix and a swelling of the matrix.

The responsive matrix may comprise a matrix of material which includesone or more monomers and/or polymers. The one or more monomers and/orpolymers contains functional groups that enable the binding moiety to beattached to or otherwise in stable association with the nanoparticle toform the conjugate. The polymer can be a natural polymer, a syntheticpolymer, a combination of natural and synthetic polymers, shape memorypolymers, block co-polymers (PEO, PPO), or derivatives of each type. Forexample, the matrix polymer may be poly (N-isopropylacrylamide). Thematrix polymer may also be (or include), for example,Poly(N-isopropylacrylamide) (PNIAAm), Poly(N,N-diethyacrylamide)(PDEAAm), P(NIAAm-co-BMA), PEO-PPO-PEO (e.g., Pluronic®),N,N-diethylaminoethyl methacrylate (DEA), 2-hydroxypropyl methacrylate(HPMA), Poly-(methacrylic acid-g-ethylen glycol),Poly(2-glucosyloxyethyl methacrylate),Poly(N-vinyl-2pyrrolidone-co-3-(acrylamido)phenylboronic acid), and/orN—(S)-sec-butylacrylamide. The functional groups can be any appropriatechemical functional group, e.g. carboxy, amino, or sulfhydryl groups. Aspecific moiety or moieties may be attached to the nanoparticle viaconjugation to these groups, or by physical adsorption and/or throughhydrogen bonds or van der Waals interactions. The responsive polymermatrix, through physical and/or chemical stimuli, mediates the specificvolume of the polymer layer, leading to a detectable change inNMR-measurable properties such as T₂ relaxivity.

“Responsive polymers” (also referred to herein as “smart polymers”) arepolymers that are, for example, sensitive to pH, ionic strength, andspecific molecular and biomolelar analytes. In these cases the hydrationlevel, cross-link density, or other characteristic of the polymerchanges in response to a changes in the sample, for example, biofluid.This change in polymer state leads to changes in the magnetic resonancesignals that can be detected by an implanted radiofrequency coil.Suitable smart polymers are known in the art, and described, forexample, in Gemeinhart, R A, Chen, J, Park, H, Park, K. 2000.pH-sensitivity of fast responsive superporous hydrogels. J. Biomater.Sci. Polym. Ed. 11: 1371-1380; Murakami, Y, Maeda, M. 2005.DNA-responsive hydrogels that can shrink or swell. Biomacromolecules, 6:2927-2929; Miyata, T, Uragami, T, Nakamae, K. 2002.Biomolecule-sensitive hydrogels. Adv Drug Deliv Rev, 54: 79-98; andZhang, R, Bowyer, A, Eisenthal, R, Hubble, J. 2006. A smart membranebased on an antigen-responsive hydrogel. Biotechnol Bioeng.

Probeheads of the present invention include a radiofrequency coil.

A “radiofrequency coil” as used herein is a is a coil that is suited tosense and/or detect magnetic resonance signals in an associateddetection volume, and, optionally, also allows to apply/emitradiofrequency pulses with associated pulse length(s) to a sample underinvestigation with the probehead as part of a magnetic resonance system.Suitable radiofrequency coil types include planar coils and “wholevolume” coils such as might be constructed of opposed saddle coils,solenoids, Helmholtz coils and the like. Typically, the probeheadsemployed in the systems of the present invention include solenoids.

“Detection volume” as used herein refers to a volume associated with agiven radiofrequency coil from which magnetic resonance signals, inprinciple, are detectable with the given radiofrequency coil as part ofa given magnetic resonance system. “Detectable” as used herein refers todistinguishable from the background noise level, that is, a magneticresonance signal is detectable if a signal can be distinguished frombackground noise level with a given radiofrequency coil as part of agiven magnetic resonance system. The detection volume for a givenradiofrequency coil-magnetic resonance system combination can becalculated, approximated and/or measured using methods known in the art.Typically, however, it is sufficient to approximate the detectionvolume. For example, for a solenoid coil, typically, the detectionvolume is effectively, the volume enclosed within the coil, which,typically, is of about cylindrical shape. In certain embodiments aradiofrequency coil is a cylinder shape. Thus, for a solenoid a goodapproximation of the detection volume is the volume of the enclosedcylinder, which can be calculated very easily. Similar approximationsare known in the art for other types of radiofrequency coils (see, e.g.,Mispelter, J., Lupu, M., Briquet, A. “NMR Probeheads for biophysical andbiomedical experiments” 2006 Imperial College Press, London.). Incertain embodiments a radiofrequency coil is wound to enclose a coilvolume having a shape of about cylindrical shape and the associateddetection volume is effectively the volume of the cylindrical shape. Insome embodiments a radiofrequency coil is positioned to have the coilvolume include between about 80 percent and about 100% of the excitablevolume. In still other embodiments a radiofrequency coil is positionedto have the coil volume include effectively all of the excitable volume.

“Sensitive volume” as used herein refers to the overlap volume betweenthe excitable volume and the detection volume, and is the volume fromwhich magnetic resonance signals can be detected with the radiofrequencycoil. A sensitive volume is determined by a fill factor (i.e., afraction of the detection volume of an RF coil which is filled with asample volume).

In some embodiments, a fill factor is between about 10 percent and about100 percent. In certain embodiments a fill factor is between about 50percent and about 100 percent. In some embodiments the fill factor isabout 80 percent. In certain embodiments the fill factor is effectively100 percent. In some embodiments a fill factor is at least about 0.1, atleast about 0.5, at least about 0.75, at least about 0.9, and or about1.

Typically, a detection volume includes between about 10 percent andabout 100 percent of the excitable volume. More typically, a detectionvolume of a given radiofrequency coil within the probehead includesbetween about 50 percent and about 100 percent of the excitable volume.Even more typically, a detection volume includes about 80 percent of theexcitable volume. Most typically, a detection volume includeseffectively all of the excitable volume.

Also, typically, an excitable volume includes between about 10 percentand about 100 percent of the detection volume. More typically, theexcitable volume includes between about 50 percent and about 100 percentof the detection volume. Even more typically, the excitable volumeincludes between about 80 percent and about 100 percent of the detectionvolume. Most typically, the excitable volume includes effectively all ofthe detection volume.

Further, for a given sample volume within the probehead, typically, thesample volume includes between about 10 percent and about 100 percent ofthe excitable volume. More typically, the sample volume includes betweenabout 50 percent and about 100 percent of the excitable volume. Evenmore typically, the sample volume includes between about 80 percent andabout 100 percent of the excitable volume. Even more typically, thesample volume includes effectively all of the excitable volume. Mosttypically, the sample volume includes effectively all of the excitablevolume and the detection volume includes effectively all of the samplevolume.

In some embodiments a sample volume includes effectively all of theexcitable volume and a detection volume includes effectively all of thesample volume. In still further embodiments a sample volume includesbetween about 10 and about 100 percent of the sensitive volume.

Typically, for a magnetic field of between about 0.2 Tesla and 1.1Tesla, radiofrequency coils with associated detection volumes of lessthan about 500 μl are used. More typically, radiofrequency coils withassociated detection volumes of less than about 100 μl are used. Evenmore typically, radiofrequency coils with associated detection volumesof less than about 10 μl are used. Even more typically, radiofrequencycoils with associated detection volumes of less than about 5 μl areused. Most typically, radiofrequency coils with associated detectionvolumes of less than about 1.6 μl are used. In some embodiments aradiofrequency coil with associated detection volume of about 1.6 μl isused, and a sample volume of about 0.4 μl is used.

A radiofrequency coil of a given probehead of the present inventionsenses and/or detects magnetic resonance signals of a sample in thepresence of a magnetic field and provides the sensed signals to aprocessing unit. The processing unit can be included within a probehead;but does not have to be included in a probehead. In any case, aprobehead contains any parts, for example, circuitry, logic circuitry,power sources and other parts such as capacitors and the like, as knownin the art, to allow the sensed signals to be provided to the processingunit. For example, a probehead of the present invention that is to beused in a magnetic resonance system with a radiofrequency coil of theprobehead being inductively coupled to the processing unit via anexternal pickup coil, typically, includes the radiofrequency coil aspart of a radiofrequency circuit with one or more tuning capacitorsincluded in the circuit. In one embodiment a probehead further comprisesat least one capacitor, wherein a radiofrequency coil and at least onecapacitor are part of a radiofrequency circuit.

One embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) at least one magnet providing amagnetic field, and (b) a radiofrequency coil having an associateddetection volume, the radiofrequency coil being adapted and positionedsuch that its detection volume includes between about 80 percent andabout 100 percent of an excitable volume, wherein the magnetic field hasa magnetic field strengths of less than about 1.1 Tesla.

Another embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) at least one permanent magnetproviding a magnetic field, and (b) a radiofrequency coil having anassociated detection volume, the radiofrequency coil being adapted andpositioned such that its detection volume includes between about 80percent and about 100 percent of an excitable volume, wherein themagnetic field has a magnetic field strengths of less than about 1.1Tesla.

Another embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) at least one permanent magnetproviding a magnetic field, and (b) a radiofrequency coil having anassociated detection volume, the radiofrequency coil being adapted andpositioned such that its detection volume includes effectively all of anexcitable volume, wherein the magnetic field has magnetic fieldstrengths of less than about 1.1 Tesla.

Another embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) at least one permanent magnetproviding a magnetic field, and (b) a radiofrequency coil having anassociated detection volume, the radiofrequency coil being adapted andpositioned such that its detection volume includes effectively all of anexcitable volume, wherein the magnetic field having magnetic fieldstrengths of less than about 1.1 Tesla, the probehead further comprisinga sample volume and the sample volume including between about 10 andabout 100 percent of the excitable volume.

Another embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) at least one permanent magnetproviding a magnetic field, and (b) a radiofrequency coil having anassociated detection volume, the radiofrequency coil being adapted andpositioned such that its detection volume includes effectively all of anexcitable volume, wherein the magnetic field having magnetic fieldstrengths of less than about 1.1 Tesla, the probehead further comprisinga sample volume, and the sample volume including between about 50 andabout 100 percent of the excitable volume.

Another embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) at least one permanent magnetproviding a magnetic field, and (b) a radiofrequency coil having anassociated detection volume, the radiofrequency coil being adapted andpositioned such that its detection volume overlaps at least partly withan excitable volume, wherein the magnetic field having magnetic fieldstrengths of less than about 1.1 Tesla, the probehead further comprisinga sample volume, the sample volume including effectively all of theexcitable volume and the detection volume including effectively all ofthe sample volume.

Another embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) at least one permanent magnetproviding a magnetic field, and (b) a radiofrequency coil having anassociated detection volume, the radiofrequency coil being adapted andpositioned such that its detection volume overlaps at least partly withan excitable volume, wherein the magnetic field has a magnetic fieldstrengths of less than about 1.1 Tesla, and wherein the probeheadfurther comprises a sample volume, the excitable volume and thedetection volume overlapping in a sensitive volume, and wherein thesample volume includes between about 10 and 100 percent of the sensitivevolume.

Another embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) at least one permanent magnetproviding a magnetic field, and (b) a radiofrequency coil having anassociated detection volume, the radiofrequency coil being adapted andpositioned such that its detection volume overlaps at least partly withan excitable volume, wherein the magnetic field having magnetic fieldstrengths of less than about 1.1 Tesla, wherein the probehead furthercomprises a structure defining a sample volume, the excitable volume andthe detection volume overlapping in a sensitive volume, and wherein thesample volume includes between about 50 and 100 percent of the sensitivevolume.

Another embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) at least one permanent magnetproviding a magnetic field, and (b) a radiofrequency coil having anassociated detection volume, the radiofrequency coil being adapted andpositioned such that its detection volume overlaps at least partly withan excitable volume, wherein the magnetic field has a magnetic fieldstrengths of less than about 1.1 Tesla, the probehead further comprisinga sample volume, the excitable volume and the detection volumeoverlapping in a sensitive volume, and wherein the sample volumeincludes effectively all of the sensitive volume.

Another embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) two permanent magnet providing amagnetic field, and (b) a radiofrequency coil having an associateddetection volume, the radiofrequency coil being adapted and positionedsuch that its detection volume overlaps at least partly with anexcitable volume, wherein the magnetic field has magnetic fieldstrengths of less than about 1.1 Tesla, wherein the radiofrequency coilis wound around a sample tube or capillary to enclose, theradiofrequency coil and the sample tube or capillary enclosing a samplevolume within the sample tube or capillary. In certain embodiments, theexcitable volume and the detection volume are overlapping in a sensitivevolume, and the sample volume includes between about 50 and about 100percent of the sensitive volume.

Another embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) two permanent magnet providing amagnetic field, and (b) a radiofrequency circuit comprising (1) aradiofrequency coil and (2) a capacitor, the radiofrequency coil havingan associated detection volume and the radiofrequency coil being adaptedand positioned such that its detection volume overlaps at least partlywith an excitable volume, wherein the magnetic field has magnetic fieldstrengths of less than about 1.1 Tesla, the radiofrequency coil beingwound around a sample tube or capillary to enclose, the radiofrequencycoil and the sample tube or capillary enclosing a sample volume withinthe sample tube or capillary, the excitable volume and the detectionvolume overlapping in a sensitive volume, and the sample volume includesbetween about 50 and about 100 percent of the sensitive volume.

Another embodiment of the present invention is a probehead for magneticresonance relaxometry that includes (a) two permanent magnet attached toa yoke providing a magnetic field, and (b) a radiofrequency circuitcomprising (1) a radiofrequency coil and (2) a capacitor, theradiofrequency coil having an associated detection volume and theradiofrequency coil being adapted and positioned such that its detectionvolume overlaps at least partly with an excitable volume, wherein themagnetic field has magnetic field strengths of less than about 1.1Tesla, the radiofrequency coil being wound around a sample tube orcapillary to enclose, the radiofrequency coil and the sample tube orcapillary enclosing a sample volume within the sample tube or capillary,the excitable volume and the detection volume overlapping in a sensitivevolume, and the sample volume including between about 50 and about 100percent of the sensitive volume. Other specific embodiments of thepresent invention are the probeheads as described in the precedingparagraphs, wherein a probehead is adapted for a pulse length of betweenabout 0.4 microseconds and about 10 microseconds, between about 1microsecond and about 4 microseconds, or between about 1.5 microsecondsand 2.5 microseconds, and, independently, each magnet is independentlyin any dimension less than about two inches, less than about 1 inch, orless than about 0.5 inch. Other embodiments of the present inventioninclude methods of preparing probeheads provided and described in thepreceding paragraphs, and the examples which follow. In one embodimentis a method for preparing a probehead for use in a magnetic resonancerelaxometer. The method comprises the steps of (a) providing at leastone magnet or magnetic field generator providing a magnetic field, (b)providing a radiofrequency coil, (c) positioning the radiofrequency coilto have its associated detection volume overlap at least partly with anexcitable volume, (d) positioning a space capable of accommodating asample volume having an associated excitable volume; and (e) adaptingthe space for the sample volume and the radiofrequency coil according toa radiofrequency pulse optimized for the magnetic field distributioncorresponding to the position of the detection volume. The probehead isthus optimized to obtain relaxometry parameters from a sample containedin the sample volume.

Another embodiment is a method of preparing a small probehead for use inportable magnetic resonance relaxometry. The method comprises the stepsof (a) attaching two magnets or two magnetic field generators to a yokesuch that the south pole surface of one of the magnets or magnetic fieldgenerators opposes the north pole surface of the other magnet ormagnetic field generator to form a gap between the magnets or magneticfield generators and to provide a magnetic field in the gap, (b)positioning a space capable of accommodating a sample volume having anassociated excitable volume, and (c) positioning a radiofrequency coilwithin the gap, the radiofrequency coil having an associated detectionvolume and being adapted to emit a radiofrequency pulse with a pulselength, the radiofrequency coil being positioned and designed to havethe detection volume at least partly overlap with an excitable volumewithin the gap. The probehead is thus optimized to obtain relaxometryparameters from a sample contained in the sample volume.

In a further embodiment a method of preparing a small probehead for usein a portable magnetic resonance relaxometer further comprises the stepof providing, calculating, and/or measuring a magnetic field map of theat least one magnet or magnetic field generator, and further wherein thestep of providing the radiofrequency coil comprises selecting ormanufacturing a radiofrequency coil dimensioned based on the magneticfield map to optimize its associated detection volume be at least aslarge as the excitable volume.

In another further embodiment a method of preparing a small probeheadfor use in a portable magnetic resonance relaxometer further comprisesthe step of providing, calculating or measuring a magnetic field map ofthe at least one magnet or magnetic field generator, and further whereinthe step of positioning the radiofrequency coil is based on the magneticfield map to optimize its associated detection volume overlap at leastpartly with the excitable volume.

In yet another further embodiment a method of preparing a smallprobehead for use in a portable magnetic resonance relaxometer furthercomprises the step of providing, calculating or measuring a magneticfield map of the at least one magnet or magnetic field generator, andfurther wherein providing the radiofrequency coil comprises selecting ormanufacturing a radiofrequency coil dimensioned based on the magneticfield map to have its associated detection volume be at least as largeas the excitable volume, and positioning the radiofrequency coil isbased on the magnetic field map to have the associated detection volumeoverlap at least partly with the excitable volume.

EXEMPLIFICATION Example 1

The probehead of FIG. 2 (also referred herein as “Abe” probehead) wasfabricated from a custom machined c-shaped yoke 208 (0.688″×0.5″×0.438″,steel), custom machined electronics enclosure 201 (1″×0.5″×0.5″,aluminum), coil holders 202 (0.19″×0.19″, teflon), magnet positioner(0.438″×0.5″×0.063″, Teflon), sample tube 203 (1 mm O.D., 0.5 mm I.D.,0.6″ long, Teflon), and ten-turn RF coil 204 (32 gage enamel coatedcopper wire, hand-wound, fastened to the sample tube by lock-titeinstant adhesive). The resonant circuit was constructed by a 10-120 pFvariable matching capacitor 206, and a combination of a 10-120 pFvariable capacitor 206 and two fixed capacitors 205, and a bulkhead SMAconnector 207.

Two inexpensive, off-the-shelf permanent magnets made of NdFeB 200 wereattached to the steel yoke 208 as shown schematically in FIGS. 1 and 5.(FIGS. 1 and 5 show two magnets 100 attached to a c-shaped yoke 101 anda radiofrequency coil 102 positioned between the magnets). The size ofthe magnets was ¼″×⅛″×½″, magnetized along the ⅛″ axis. The largestdimension of the array, including the supporting yoke was ½″. It isbelieved that the steel yoke helps driving the magnetic flux between themagnet blocks, following the path of high magnetic permeance.

The magnetic field was mapped by measuring along the three axis aroundthe geometrical center of the gap between the magnet pieces. FIGS. 6 and7 show the magnetic field distribution.

A solenoid radiofrequency coil 204 was designed and positioned based onthe magnetic field map with the goal of maximizing the sensitive volume,(e.g., the overlap volume of excitable volume and detection volume to beexcited by short duration RF pulses). This was achieved by winding theradiofrequency coil to enclose a volume determined by the bandwidth ofthe RF pulses, provided in FIG. 6 as box shaped area 600, that is,approximately 1 mm diameter by about 2 mm length. The radiofrequencycoil was fabricated using these dimensions. In this manner, a coiltaking up about 20% of the gap between the magnets with a microlitersensitive volume was achieved. It is contemplated that this concept canbe further exploited by, e.g., using more than one block at each side ofthe magnet. This allows for higher control on the field distribution andtherefore a further size reduction keeping the same sensitive volume.

The Abe probehead was attached to a KEA spectrometer (not shown;Magritek, Wellington, New Zeeland) outfitted with a Tomco pulseamplifier and controlled by Prospa softare (Magritek, Wellington, NewZeeland). The resonant circuit was tuned at the so-called Larmorfrequency by using the “wobble” macro provided by the Prospa software,that is, using standard procedures known in the art and signal wasacquired utilizing a conventional CPMG pulse sequence as controlled bythe “cpmgadd” and “cpmgint” macros of the Prospa software, that is, byusing standard procedures known in the art. FIG. 8 shows the signaldecay utilizing a conventional CPMG pulse sequence.

The sample was loaded by means of a syringe outfitted with a fusedsilica glass capillary. A series of CuSO₄ samples were analyzed as wellas nanoparticle assay solutions. The nanoparticle assay solutionsconsisted of antibody functionalized nanoparticles that bind to the betasubunit of the hCG protein (Kim, G. Y., Josephson, L., Langer, R., Cima,M. J. “Magnetic Relaxation Switch Detection of Human ChorionicGonadotrophin”. 2007, Bioconjugate Chemistry 18(6), 2024-2028.). Twosolutions were prepared that contained 0.14 mM nanoparticle iron and 0and 1 μg/mL beta subunit of hCG in PBS pH 7.4 buffer. The T₂ values ofthese solutions were measured in a volume of 300 μL on a Bruker minispecand in a volume of 0.4 μL on the Abe probehead. Both measurements showeda decrease in T₂ upon addition of hCG. However, the absolute T₂ valueson the Abe magnet were lower than those for the Bruker minispec. It isbelieved that that the reason for lower values is that the T₂ valuemeasured with the Abe probehead is an effective T₂ value that includeseffects from diffusion, temperature, and stimulated echoes. However, themost important result is that addition of hCG leads to a change in themeasured T₂ and this information is successfully provided using the manyorders of magnitude less expensive and smaller Abe probehead.

Example 2

Magnet configuration and yoke design can be accomplished initially by atheoretical prediction of what magnet and yoke configuration will leadto in terms of magnetic field strength. This was done by using standardanalytical methods. A magnet assembly including two NdFeB permanentmagnets (1″×1″×0.5″) 301 was fabricated according to this method (see“T2-yoke” in FIG. 3). The yoke 300 was fabricated from steel stock usingstandard machining methods. The magnetic field in the x, y, and zdirections was determined by fixing a gaussmeter probe relative to themagnet and moving the magnet in incremental steps with a three axisstage while recording the field strength as a function of position. Thestrength along the x, y, and z axes was measured by fixing two of thethree directions to zero while incrementing the other. The same processwas conducted for a pre-fabricated Halbach magnet (FIG. 4; the figureshows the Halbach magnet 400 while the magnetic field is being measuredwith a Gaussmeter probe 401).

Values obtained for a field map of each of the T2-yoke and the Halbachmagnet configurations were plotted as function of position and magneticfield strength as shown in FIG. 9. Data were fitted with a quadraticfunction (y=ax²+bx+c). The information of these plots was used to designa radiofrequency coil by determining what length in each dimensioncorresponds to a region that can be excited by a 2 μs RF pulse. Forexample, a 2 μs pulse excites a bandwidth of 500 kHz(bandwidth=(pulselength)⁻¹). For the T2-yoke shown in FIG. 3, thehomogeneous region has a field strength of 535 millitesla. The resonantfrequency of hydrogen nuclei at this field can be calculated fromf=γB _(o)/2π  (1)where f is the resonant frequency in Hz, γ the gyromagnetic ratio for ¹Hnuclei (267.522×10⁶ rad s⁻¹ T⁻¹), and B_(o) the magnetic field in Tesla.Accordingly, the resonant frequency for the T2-yoke is 22.8 MHz.Equation 1 can also be used to determine the range of magnetic fieldover which a sample can be excited. Solving for B_(o) and substituting500 kHz for f, a ΔB_(o) of 11 millitesla is calculated. This value wasused to determine the length in each dimension for the volume of samplethat can be excited by a 2 μs pulse. FIG. 9 shows a graphicalrepresentation as to how this can be determined for each magnet. A boxwith a height corresponding to 11 mT is positioned on the plot such thatone edge is at the minimum of the curve fit for “along the gap” and theother edge is used to determine the appropriate width of the box suchthat the two corners are traversed by the curve fit. The width of thisbox (˜5 mm) corresponds to the length in this dimension that anexcitation coil would enclose to maximize the sensitive volume. Asimilar box is shown for the Halbach magnet for the “bottom to top”dimension. Other analytical methods can be used to determine this, butthe general idea is taking the ΔB_(o) and using the field map totranslate that into a distance for each dimension.FIGS. 10 and 11 show radiofrequency circuits that were fabricated forthe Halbach magnet and the T2-yoke respectively to form a completeprobehead. The coils 1000 were custom made from inductors that arecommercially available (inductors can be hand wound as for the previousexample), enclose a sample volume to which sample can be deliveredthrough a sample tube 1001, are part of a radiofrequency circuit thatincludes capacitors 1003 and a bulkhead SMA connector 1002, and aresupported by a support plate 1004. Magnetic resonance signal wassuccessfully measured using these probeheads (data not shown).

What is claimed is:
 1. A method for detecting a sample characteristic ina sample using T2 relaxometry parameters comprising: a) obtaining asample having a sample characteristic; b) incubating the sample with asensing agent; c) placing the sample in a probehead having two permanentmagnets connected by a steel c-shaped yoke, the south pole surface ofone of the permanent magnets opposing the north pole surface of theother permanent magnet to form a gap and to provide an inhomogeneousmagnetic field in the gap at a strength of less than 1.1 Tesla and ahomogeneity between 50 ppm and 5000 ppm; a space capable ofaccommodating a sample volume having an associated excitable volume; anda radiofrequency coil positioned partly or completely within the gap,said radiofrequency coil wound in a cylindrical shape to enclose avolume having a diameter of approximately 1 mm and a length ofapproximately 2 mm, the radiofrequency coil being positioned accordingto a magnetic field map such that its detection volume overlaps at leastpartly with the excitable volume, wherein the space accommodating thesample volume and the radiofrequency coil is adapted and positionedaccording to a radiofrequency CPMG pulse bandwidth optimized for amagnetic field distribution corresponding to a position of the samplevolume; and wherein the probehead is optimized to obtain T2 relaxometryparameters from a sample contained in the detection volume; d) exposingthe sample to a radiofrequency pulse; and e) obtaining T2 relaxometryparameters from said sample, thereby detecting said samplecharacteristic.
 2. The method of claim 1, wherein said samplecharacteristic is a chemical property or physical property of thesample.
 3. The method of claim 2, wherein said chemical property orphysical property of the sample is a concentration of an analyte,pH-value, ionic strength, or hydration state.
 4. The method of claim 3,wherein said analyte is selected from the group consisting of amolecule, an ion, and a radical.
 5. The method of claim 4, wherein saidmolecule is selected from the group consisting of a protein, a peptide,a polypeptide, an amino acid, a nucleic acid, an oligonucleotide, atherapeutic agent, a metabolite of a therapeutic agent, RNA, DNA, andantibody, an organism, a virus, a bacteria, a carbohydrate, apolysaccharide, glucose, a lipid, a gas, an electrolyte, a lipoprotein,cholesterol, a fatty acid, a glycoprotein, a proteoglycan, and alipopolysaccharide.
 6. The method of claim 1, wherein said sensing agentis selected from the group consisting of dry reagent compositions,magnetic particles, responsive polymers, and magnetic resonance contrastagents.
 7. The method of claim 6, wherein said sample characteristic isa concentration of an analyte and said magnetic particles aresuperparamagnetic nanoparticles that are functionalized with bindingmoieties able to bind said analyte.
 8. The method of claim 7, whereinsaid analyte is selected from the group consisting of an amino acid, anucleic acid, an oliogonucleotide, a therapeutic agent, a metabolite ofa therapeutic agent, a peptide, a polypeptide, a protein, acarbohydrate, a polysaccharide, a virus, bacteria, and a polymer.
 9. Themethod of claim 7, wherein said binding moieties are operational toalter aggregation of the magnetic particles as a function of thepresence or concentration of the analyte.
 10. The method of claim 7,wherein said superparamagnetic particles are 1 nm to 5 μm in diameter.11. The method of claim 7, wherein the functionalized superparamagneticparticles further comprise a polymer coating and the concentration ofsaid analyte is detected without aggregation of said superparamagneticparticles.
 12. A method for preparing a probehead for use in magneticresonance relaxometry comprising: a) providing a probehead having twopermanent magnets connected by a steel c-shaped yoke, the south polesurface of one of the permanent magnets opposing the north pole surfaceof the other permanent magnet to form a gap and to provide aninhomogeneous magnetic field in the gap at a strength of less than 1.1Tesla and a homogeneity between 50 ppm and 5000 ppm; and a space capableof accommodating a sample volume having an associated excitable volume;b) providing a magnetic field map of the inhomogeneous magnetic field;c) based on the magnetic field map, positioning a radiofrequency coilpartly or completely within the gap, said radiofrequency coil wound in acylindrical shape to enclose a volume having a diameter of approximately1 mm and a length of approximately 2 mm, the radiofrequency coil beingpositioned such that its detection volume overlaps at least partly withthe excitable volume, wherein the space accommodating the sample volumeand the radiofrequency coil is adapted and positioned according to aradiofrequency CPMG pulse bandwidth optimized for a magnetic fielddistribution corresponding to a position of the sample volume; whereinthe probehead is optimized to obtain T2 relaxometry parameters from asample contained in the detection volume, thereby preparing a probeheadfor use in magnetic resonance relaxometry.
 13. The method of claim 12,wherein each of the two permanent magnets are about 1 inch or less inany dimension.
 14. The method of claim 12, wherein the inhomogeneousmagnetic field is between about 0.2 and 0.8 Tesla.
 15. The method ofclaim 12, wherein the radiofrequency coil provides a pulse lengthbetween about 0.4 μs and about 10 μs.
 16. The method of claim 12,wherein the detection volume comprises 80-100% of the excitable volume.17. The method of claim 12, wherein the radiofrequency coil occupiesabout 20% of the gap between the magnets.
 18. The method of claim 1 or12, wherein the magnetic field has a homogeneity between 100 ppm and1000 ppm.